README.md

Sprockets: Rack-based asset packaging

Sprockets is a Ruby library for compiling and serving web assets.
It features declarative dependency management for JavaScript and CSS
assets, as well as a powerful preprocessor pipeline that allows you to
write assets in languages like CoffeeScript, Sass, SCSS and LESS.

Installation

Install Sprockets from RubyGems:

$ gem install sprockets

Or include it in your project's Gemfile with Bundler:

gem 'sprockets', '~> 2.0'

Understanding the Sprockets Environment

You'll need an instance of the Sprockets::Environment class to
access and serve assets from your application. Under Rails 3.1 and
later, YourApp::Application.assets is a preconfigured
Sprockets::Environment instance. For Rack-based applications, create
an instance in config.ru.

The Sprockets Environment has methods for retrieving and serving
assets, manipulating the load path, and registering processors. It is
also a Rack application that can be mounted at a URL to serve assets
over HTTP.

The Load Path

The load path is an ordered list of directories that Sprockets uses
to search for assets.

In the simplest case, a Sprockets environment's load path will consist
of a single directory containing your application's asset source
files. When mounted, the environment will serve assets from this
directory as if they were static files in your public root.

The power of the load path is that it lets you organize your source
files into multiple directories -- even directories that live outside
your application -- and combine those directories into a single
virtual filesystem. That means you can easily bundle JavaScript, CSS
and images into a Ruby library and import them into your application.

Manipulating the Load Path

To add a directory to your environment's load path, use the
append_path and prepend_path methods. Directories at the beginning
of the load path have precedence over subsequent directories.

In general, you should append to the path by default and reserve
prepending for cases where you need to override existing assets.

Accessing Assets

Once you've set up your environment's load path, you can mount the
environment as a Rack server and request assets via HTTP. You can also
access assets programmatically from within your application.

Logical Paths

Assets in Sprockets are always referenced by their logical path.

The logical path is the path of the asset source file relative to its
containing directory in the load path. For example, if your load path
contains the directory app/assets/javascripts:

Asset source file

Logical path

app/assets/javascripts/application.js

application.js

app/assets/javascripts/models/project.js

models/project.js

In this way, all directories in the load path are merged to create a
virtual filesystem whose entries are logical paths.

Serving Assets Over HTTP

When you mount an environment, all of its assets are accessible as
logical paths underneath the mount point. For example, if you mount
your environment at /assets and request the URL
/assets/application.js, Sprockets will search your load path for the
file named application.js and serve it.

Under Rails 3.1 and later, your Sprockets environment is automatically
mounted at /assets. If you are using Sprockets with a Rack
application, you will need to mount the environment yourself. A good
way to do this is with the map method in config.ru:

Accessing Assets Programmatically

You can use the find_asset method (aliased as []) to retrieve an
asset from a Sprockets environment. Pass it a logical path and you'll
get a Sprockets::BundledAsset instance back:

environment['application.js']
# => #<Sprockets::BundledAsset ...>

Call to_s on the resulting asset to access its contents, length to
get its length in bytes, mtime to query its last-modified time, and
pathname to get its full path on the filesystem.

Using Engines

Asset source files can be written in another language, like SCSS or
CoffeeScript, and automatically compiled to CSS or JavaScript by
Sprockets. Compilers for these languages are called engines.

Engines are specified by additional extensions on the asset source
filename. For example, a CSS file written in SCSS might have the name
layout.css.scss, while a JavaScript file written in CoffeeScript
might have the name dialog.js.coffee.

Styling with Sass and SCSS

Sass is a language that compiles to CSS and
adds features like nested rules, variables, mixins and selector
inheritance.

If the sass gem is available to your application, you can use Sass
to write CSS assets in Sprockets.

Sprockets supports both Sass syntaxes. For the original
whitespace-sensitive syntax, use the extension .css.sass. For the
new SCSS syntax, use the extension .css.scss.

Styling with LESS

LESS extends CSS with dynamic behavior such as
variables, mixins, operations and functions.

If the less gem is available to your application, you can use LESS
to write CSS assets in Sprockets. Note that the LESS compiler is
written in JavaScript, and at the time of this writing, the less gem
depends on therubyracer which embeds the V8 JavaScript runtime in
Ruby.

To write CSS assets with LESS, use the extension .css.less.

Scripting with CoffeeScript

CoffeeScript is a
language that compiles to the "good parts" of JavaScript, featuring a
cleaner syntax with array comprehensions, classes, and function
binding.

If the coffee-script gem is available to your application, you can
use CoffeeScript to write JavaScript assets in Sprockets. Note that
the CoffeeScript compiler is written in JavaScript, and you will need
an ExecJS-supported runtime
on your system to invoke it.

To write JavaScript assets with CoffeeScript, use the extension
.js.coffee.

JavaScript Templating with EJS and Eco

Sprockets supports JavaScript templates for client-side rendering of
strings or markup. JavaScript templates have the special format
extension .jst and are compiled to JavaScript functions.

When loaded, a JavaScript template function can be accessed by its
logical path as a property on the global JST object. Invoke a
template function to render the template as a string. The resulting
string can then be inserted into the DOM.

Sprockets supports two JavaScript template languages:
EJS, for embedded
JavaScript, and Eco, for
embedded CoffeeScript. Both languages use the familiar <% … %>
syntax for embedding logic in templates.

If the ejs gem is available to your application, you can use EJS
templates in Sprockets. EJS templates have the extension .jst.ejs.

If the eco gem is available to your application, you can use Eco
templates in Sprockets. Eco
templates have the extension .jst.eco. Note that the eco gem
depends on the CoffeeScript compiler, so the same caveats apply as
outlined above for the CoffeeScript engine.

Invoking Ruby with ERB

Sprockets provides an ERB engine for preprocessing assets using
embedded Ruby code. Append .erb to a CSS or JavaScript asset's
filename to enable the ERB engine.

Note: Sprockets processes multiple engine extensions in order from
right to left, so you can use multiple engines with a single
asset. For example, to have a CoffeeScript asset that is first
preprocessed with ERB, use the extension .js.coffee.erb.

Ruby code embedded in an asset is evaluated in the context of a
Sprockets::Context instance for the given asset. Common uses for ERB
include:

embedding another asset as a Base64-encoded data: URI with the
asset_data_uri helper

inserting the URL to another asset, such as with the asset_path
helper provided by the Sprockets Rails plugin

embedding other application resources, such as a localized string
database, in a JavaScript asset via JSON

embedding version constants loaded from another file

See the Helper Methods section for more information about
interacting with Sprockets::Context instances via ERB.

String Interpolation Syntax

If you need access to Ruby from an asset but cannot use ERB's <% …
%> syntax, Sprockets also supports Ruby string interpolation syntax
(#{ … }) with the .str engine extension.

Managing and Bundling Dependencies

You can create asset bundles -- ordered concatenations of asset
source files -- by specifying dependencies in a special comment syntax
at the top of each source file.

Sprockets reads these comments, called directives, and processes
them to recursively build a dependency graph. When you request an
asset with dependencies, the dependencies will be included in order at
the top of the file.

The Directive Processor

Sprockets runs the directive processor on each CSS and JavaScript
source file. The directive processor scans for comment lines beginning
with = in comment blocks at the top of the file.

The first word immediately following = specifies the directive
name. Any words following the directive name are treated as
arguments. Arguments may be placed in single or double quotes if they
contain spaces, similar to commands in the Unix shell.

Note: Non-directive comment lines will be preserved in the final
asset, but directive comments are stripped after
processing. Sprockets will not look for directives in comment blocks
that occur after the first line of code.